The present invention relates to a solid-state image sensor of a MOS type structure which picks up, in amplified form, signal charges obtained by photodiodes of a photoelectric conversion section and, more in particular, a MOS type solid-state image sensor having two-dimensional array of row and column unit cells over a semiconductor substrate and including a photoelectric conversion section and signal scanning circuit section.
As the solid-state image sensor there have been conventionally known mainly a charge coupled device type solid-state image sensor and MOS type solid-state image sensor.
The MOS type solid-state image sensor in which a signal charge detected at a photoelectric conversion section (photodiode), at each pixel (unit cell), is amplified by a transistor has the features of a high sensitivity, single power supply driving and low dissipation power. In more detail, the potential of a signal charge storage section is modulated by a signal charge generated by the photoelectric conversion and the amplification factor of the amplifying transistor in the pixel is modulated by the potential. Since such an amplification function is possessed in the pixel, the MOS type solid-state image sensor has been expected as the solid-state image sensor suitable to the reduction of pixel size against an increase in the number of pixels and reduction in image size.
FIG. 1 is a diagrammatic view showing a cross-sectional structure of a conventional MOS type solid-state image sensor. In FIG. 1, reference numeral 101 shows a p type substrate; 102 a photoelectric conversion section; 103 an amplifying transistor; 104 an address transistor; 105 a field oxide film for element isolation; 106 a photodiode; 107 a signal reading-out gate of a reading-out transistor; 108 an amplifying gate; 109 an address gate; 110 a drain line; 111 a signal line; 112 a light shielding film; and 113 a light condensing lens.
This kind of MOS type solid-state image sensor and above-mentioned CCD type solid-state image sensor have a common feature in that photoelectric conversion is effected at the photodiode formed at an Si substrate interface. In the CCD type solid-state image sensor, electrons as photoelectrically converted signal charges are transferred in a diffusion region (signal transfer section) formed in the same Si substrate interface, whereas, in the MOS type solid-state image sensor, photoelectrically converted electrons are transmitted in connection lines. Therefore, when strong light is incident on respective pixels, there occurs, in the CCD type solid-state image sensor, a phenomenon called xe2x80x9csmearingxe2x80x9d emerging as vertical bands on a reproduced image due to the leaking, into the signal transfer section, of electrons overflowed from the photodiode and a phenomenon called xe2x80x9cbloomingxe2x80x9d which emerges as a spread image resulting from the overflowed electrons leaking into an adjacent pixel. For the case of a single-plate CCD type solid-state image sensor, overflowed electrons leak into the adjacent pixel, thus causing the so-called xe2x80x9ccolor mixingxe2x80x9d, that is, the deterioration of a color reproduction resulting from a mixing with an adjacent different color filter signal.
For the case of the MOS type solid-state type image sensor, on the other hand, xe2x80x9csmearingxe2x80x9d never occurs because the signal transfer section is not present in the substrate. However, there still occur the xe2x80x9cbloomingxe2x80x9d and xe2x80x9ccolor mixingxe2x80x9d resulting from the leaking of overflowed electrons into the adjacent pixel or the xe2x80x9cbloomingxe2x80x9d and xe2x80x9ccolor mixingxe2x80x9d resulting from signal leaking from a deeper area in the substrate.
Further, in the MOS type solid-state image sensor, a signal scanning circuit section (the amplifying transistor 103, address transistor 104, etc. in FIG. 1) is present between the photodiodes and, in comparison with the CCD type solid-state image sensor, the MOS type solid-state image sensor is favorable against the signal leaking between the adjacent pixels. In a recent public acceptance of an electronic still camera, etc., there is a growing demand for a low-cost but high performance MOS type solid-state image sensor and an improvement in the MOS type solid-state image sensor is more requested in terms of the device characteristics.
Further, the MOS type solid-state image sensor has an advantage in that various kinds of circuits, such as a photoelectric conversion section, signal scanning circuit section, peripheral circuits (a register circuit, timing circuit), A/D (Analog to Digital) converter, command circuit, D/A (Digital to Analog) converter, and DSP (Digital Signal Processor), can be formed one at a time on the same substrate. This ensures the advantage of forming a small system in the MOS type solid-state image sensor and offering a low cost product (system). When, however, many kinds of circuits are set over the same substrate, the following problem arises. That is, in the case where many kinds of circuits are so set, the analog circuit and digital circuit are formed in a mixed way and, in this case, there is a fear that the clock noise of the digital circuit will produce analog waveform noise and that the oscillation of the substrate by the digital clock will produces analog waveform noise.
It is accordingly an object of the present invention to provide a solid-state image sensor of a MOS type structure which can prevent signal charges which are generated at a deeper area of a substrate from leaking into an adjacent pixel and suppress the phenomena xe2x80x9cbloomingxe2x80x9d and xe2x80x9ccolor mixingxe2x80x9d.
Another object of the present invention is to provide a solid-state image sensor of a MOS type structure which, in a case where many kinds of circuits are mixedly mounted at the same substrate, can prevent noise in a digital circuit, as well as the oscillation of the substrate, which would exert any adverse effect on an analog waveform.
In order to achieve the object of the present invention, there is provided a solid-state image sensor of a MOS type structure which comprises an n type semiconductor substrate, at least one first p well area provided in a surface portion of the n type semiconductor substrate, a plurality of second p well areas selectively provided at a surface portion of the first p well area and higher in p type impurity concentration than the first p well area, an image pickup area comprising a two-dimensional array of row and column unit cells having a photoelectric conversion section provided in the first p well area and a signal scanning circuit section in the second p well area, and a plurality of signal lines respectively reading out signal charges from the unit cells in the image pickup area.
There is further provided a solid-state image sensor of a MOS type structure comprising an n type semiconductor substrate, a plurality of p well areas selectively provided in a surface portion of the n type semiconductor substrate, an image pickup area comprising a two-dimensional array of row and column unit cells having a photoelectric conversion section formed at the surface portion of the n type semiconductor substrate and a signal scanning circuit section formed in the p well area, a plurality of signal lines respectively reading out signal charges from the unit cells of the image pickup area, a p type buried area provided at a base portion of the n type semiconductor substrate and higher in p type impurity concentration than the p well area, and an element isolation area respectively provided at the surface of the n type semiconductor substrate at least corresponding to the boundary area of the unit cell-to-unit cell area and an element isolation p type area formed from the surface of the n type semiconductor substrate along the element isolation area to a depth area deeper than the depth of the photoconductive conversion section and higher in p type impurity concentration than the p well area.
According to the solid-state image sensor of a MOS type structure of the present invention, it is possible to, as the semiconductor substrate, an n type substrate and, by doing so, those signal charges generated at a deeper area of the semiconductor substrate can be discharged toward the substrate""s ground side.
Since, in particular, the image pickup area is formed in the first p well area and signal scanning circuit section is formed in the second p well area, those signal charges generated at the deeper area of the substrate can be positively prevented from leaking into an adjacent pixel.
In the case where the element isolation p type area is formed along the element isolation area, more positive isolation can be achieved between adjacent pixels.
Further, in the case where the p type buried area is formed at the boundary area between the n type substrate and the p well area, it is possible to more positively prevent a leakage of signals coming from a deeper area of the substrate.
In the case where the first p well area is formed with a concentration gradient of a gradually increasing impurity concentration it is possible to prevent an excessive discharge of signal charges by the n type substrate.
Further, there is also provided a solid-state image sensor of a MOS type structure according to the present invention comprising an n type semiconductor substrate, at least one first p well area provided in a surface portion of the n type semiconductor substrate and having a plurality of second p well areas, at least one third p well area provided in a surface portion of the n type semiconductor substrate and spaced a predetermined distance from the first p well area, an image pickup area comprising a two-dimensional row and column unit cells formed at the first p well area and including a photoelectric conversion section and signal scanning circuit section, a plurality of signal lines respectively reading out signal charges from the unit cells in the image pickup area, and a peripheral circuit area formed in the third p well area.
According to the solid-state image sensor of a MOS type structure according to the present invention, the image pickup area (photoelectric conversion section and signal scanning circuit section) and its peripheral area can be formed in different p well areas. By doing so it is possible to isolate many kinds of circuits, in particular, isolate the image pickup area as an analog circuit and a digital circuit of the peripheral circuit area.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.